An unsafe condition exists if a circuit breaker can be energized without a trip unit installed. The possibility of undesired closure, and subsequent energization is inherent in circuit breakers with interchangeable tripping units.
To minimize the risk of inadvertent closure of a circuit breaker, interlocking mechanisms of some complexity have been devised which generally require adjustment during assembly, some form of manual locking motion to assure secure installation, and some thought prior to assembly. Usually the interlock blocks the action of closing the circuit breaker through an interposing member or causes the rapid discharge of the closing energy to be diverted from the closing links preventing contact closure.
Circuit breakers of the type contemplated herein are used to protect electrical distribution circuits and apparatus. Electronic trip units provide tripping parameters such as overcurrents and time delays. However, it is important to prevent operation of the circuit breaker in the absence of a properly rated trip unit. By way of example, a system of this type is shown in U.S. Pat. No. 4,000,478, entitled "Static Trip Molded Case Circuit Breaker Including Trip Interlock," issued Dec. 28, 1976, to Jencks, et al.
Typically, circuit breakers require the presence of a trip unit before the circuit breaker system can be operated. For example, a circuit breaker may include a trip unit which is located in a housing recess by guide pins mounted on a plate in the housing. Corresponding holes are provided in the end of the trip unit to matingly engage the guide pins. The pins and holes are arranged in identical patterns such that the pins are received by the holes. Circuit interrupters having different electrical characteristics have rejection pins arranged in patterns different from the holes. The pin patterns and hole patterns for compatible circuit interrupters and trip units are the same, such that only trip units having circuitry compatible with the particular circuit interrupter can be properly inserted in the housing.